This invention relates to an image forming apparatus such as a full color copying machine or color printer.
Conventionally, as an image forming apparatus which outputs a color image, a so-called 4-series tandem type full color copying machine in which four image forming units configured to form toner images of respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) based on image signals subjected to color separation are arranged along a conveyor belt is known.
Each of the image forming units of respective colors includes a photosensitive drum arranged to roll in contact with the conveyor belt, an electrifying device which charges the drum surface to preset potential, an exposure device which exposes the drum surface to form an electrostatic latent image thereon, a developing device which supplies toner to the electrostatic latent image on the drum surface to develop the same, and a transfer device which transfers a thus developed toner image onto recording paper attracted to and fed by the conveyor belt. Thus, recording paper attracted to the conveyor belt is fed through the four image forming units (process units), toner images of respective colors are transferred onto the recording paper in a superposed fashion, and then the recording paper is fed to a fixing device in which the toner images of respective colors are fixed on the recording paper to form a color image.
The above 4-series tandem type full color copying machine includes a scanner section, image processing section and printer section.
As the 4-series drum configuration of the printer section, four process units and photosensitive members of Y-M-C-BK are physically separated and arranged. For example, if the distance is 75 mm, the separation distance of 75 mm in the case of 600 dpi indicates that the above units are arranged with a separation of 600 dpi/25.4 dots/mm*75 mm=1771 lines. That is, 600÷(coefficient of conversion from inch to millimeter)xc3x9775
=600+25.4xc3x9775
=1771 (lines).
The image signals are input from the image processing section to the printer section as Y, M, C, BK signals in correspondence to Y-M-C-BK of the process units.
In this case, the configuration can be made such that a laser positioning signal used to control the laser output position can be input to the printer section together with the image signals.
The Y, M, C, BK image signals input from the image processing section are used to form respective images according to the image forming timing of the printer section.
In the image forming timing in the printer section, the image forming timing in the sub-scanning direction is explained.
In the conventional monochrome copying machine, an image is formed by synchronizing the paper feeding timing of the printer section with the read scan start timing and starting the scanning process at timing such that a toner image formed on the photosensitive drum may form an image in a preset position of paper fed.
In the image forming process in the 4-series tandem engine, the image forming timing for yellow is determined by adjusting the paper feeding timing and scan timing like the conventional monochrome image forming timing in a case where images are formed on the paper in an order of Y, M, C, BK (which are abbreviation of yellow, magenta, cyan, black).
On the other hand, in the image forming process for other colors of M, C, BK in the 4-series tandem engine, it is required to superpose images on the yellow image and it is necessary to strictly perform timing control in the main scanning direction and sub-scanning direction.
The superposition technique in the main scanning direction and sub-scanning direction is called a registration technique and the timing control process in the main scanning direction is performed by use of the known technique.
In this case, image formation timing control of Y, M, C, BK in the 4-series drums in the sub-scanning direction is explained.
In the conventional image forming system in the 4-series drum system, generally, superposition or registration of image data items for the respective drums can be attained by inputting all color signals of Y, M, C, BK to a delay memory controller at the same timing and delaying respective image data items of M, C, BK according to the distances between the drums of Y-M, Y-C, Y-BK in the delay memory controller.
The drumxe2x80x94drum distance is expressed by the number of sub-scanning lines based on the sub-scanning resolution, the number of sub-scanning lines is counted from the yellow image forming timing set as the starting point, and image data of magenta color is delayed by the memory and output so that the sub-scanning deviation amount (the number of output timings) of magenta color with respect to yellow color will become equal to the drumxe2x80x94drum distance (the number of sub-scanning lines corresponding to the drumxe2x80x94drum distance) (for example, if the drumxe2x80x94drum distance is 75 mm, the image data is output with the delay of 1771 lines).
The output image signal is input to the laser control section of the printer section and used as an ON/OFF signal of laser to form an image on the photosensitive drum.
In the same manner as described above, image data items of C, BK are delayed by the sub-scanning deviation amounts (3542 lines, 5313 lines) between the drums of Y-C, Y-B and output, and consequently, four colors of Y, M, C, BK are output and superposed. Thus, a method for attaining registration of four colors by use of the engine of 4-series drum configuration is known as a general method.
The measurement and adjustment of the drumxe2x80x94drum distances of respective colors are attained by measuring the distances by use of the known sub-scanning registration technique and feeding back the measured data to the number of sub-scanning lines.
With the above method, a sub-scanning counter is counted up with the yellow image input timing set as a reference and image data of magenta is output so that the count of the sub-scanning counter will become equal to the output timing of magenta color. Therefore, when the process for the longest drumxe2x80x94drum distance of Y-BK is performed, image data of BK is kept stored in the memory until the output timing of BK color is attained after the image data items of Y, M, C, BK were input and then the image data of BK is started to be output at the output start timing of BK in many cases.
Since the sub-scanning deviation amount is counted because of correction of sub-scanning delay even after the input image data effective area comes to an end, there occurs a possibility that the image forming position is shifted from an original position when a print in which input timing of the first page and input timing of the second page are close to each other is made.
Specifically, at the time of high-speed image printing, if an image is input at output timing at which a yellow image is newly input before the sub-scanning count timing of M, C, BK is terminated (before M, C, BK colors start to be output), the counts of the M, C, BK color delay counters are cleared, timings deviated from the original counts are counted, and consequently, a problem that the image forming position is shifted from the correct position occurs. Generally, the process for inputting the image at such timing is inhibited and it is necessary to terminate outputting of the images of M, C, BK colors when the first and second pages are input because of the above restriction. Therefore, the above method is not suitable for the high-speed printing.
An object of this invention is to provide a 4-series tandem type full color copying machine capable of attaining a high-speed process.
In order to attain the above object, an image forming apparatus of this invention comprises a supply section which supplies image data items of different colors; an image forming section including a feeding section which feeds a recording medium, a plurality of image forming units which respectively have rotatable photosensitive drums arranged side by side on the feeding section and transfer respective image data items of different colors onto the recording medium fed by the feeding section, and an output section which outputs sync signals in a sub-scanning direction in synchronism with the processes by the respective image forming units; and a delay section which delays image data items of respective colors from the supply section by feeding times of the recording medium corresponding to intervals between the respective image forming units based on the sync signals from the output section of the image forming section and outputs the image data items to the image forming section.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.